Process of bright dipping zinc base alloys



United States Patent PROCESS OF BRIGHT DIPPING ZINC BASE ALLOYS Kenneth Raymond Hampel, Grandville, Mich., assignor to Jervis Corporation, Grandville, Mich., a corporation of Michigan No Drawing. Application June 23, 1954 Serial No. 438,879

6 Claims. (CI. 41-42) The present invention relates to a deburring and bright dipping or chemical polishing process for zinc alloys, containing upwards of 85% by weight of zinc, particularly for zinc base die castings. It is a process intended particularly for the chemically bright dipping of alloys in which zinc is the principal ingredient. A bright, lustrous finish results when subjecting zinc alloys to the action of an acid aqueous solution, the essential active polishing ingredients thereof being, hydrofluoric acid (HF), chromic acid (H CrO and nitric acid (HNO The invention works particularly well on zinc base castings such as are made of an alloy known commercially as Zamak, as described in Condensed Chemical Dictionary, 3rd edition (1942), page 681, where they are described as alloys #2, #3 and #5. The chemical composition of these alloys is as follows:

The invention by no means is restrictive to these three named alloys, but covers all alloys in which the principal metal ingredient is zinc. However, it probably will find its greatest use in the bright dipping of Zamak #3 alloy, as this alloy is most commonly used in the United States.

Prior to the coming into being of the present invention, there were no direct methods for chemically brightening zinc base die castings. There are methods in use whereby the zinc base castings may be dipped in an acid chromate solution which produces a brown chromate film on the articles, which film, however, must be removed by means of a substantially applied alkaline solution before the casting is brightened. In some instances the castings are zinc plated and then brightened. There are numerous solutions in use for bright dipping pure zinc; i.e., zinc electrodeposits. Of course, the usual method of brightening zinc alloys has been to wheel-buff or barrel-tumble them with suitable abrasives. Wheel-butfing or polishing is expensive and is not always adaptable to articles having intricate shapes or deep recesses.

In the present invention the ratio of chronric acid and hydrofluoric acid is important. If the hydrofluoric acid is 08 035 BEEREML Patented Sept. 15, 1959 acid aqueous solution composition and a method for the chemical brightening of zinc alloys.

A further object is to provide a chemical solution for treating zinc alloys, which is characterized by economy in use over other methods, short immersion times (10-60 seconds) to obtain the bright finish, and ease of application.

Still a further object is to provide a bright, passive surface which is receptive to painting and electroplating operations.

Yet another object of my invention is to provide a bath which is not extremely critical in regard to composition or to temperature of operation.

Still another object is to provide a chemical bath to remove the grey oxidation film from zinc alloys and impart thereto a brilliant lustrous surface.

Other objects will be evident from uses and claims cited elsewhere in this patent disclosure.

The present invention, which provides a bright surface by a single dip method, does not provide quite the luster on zinc alloys that is possible on pure zinc itself by other dips adapted for pure zinc. This is due to the fact that zinc alloys, particularly castings, have other alloy constituents which affect the brightness. Then, too, cast zinc alloys have varying grain sizes, different grain compositions, etc., which tend to lessen the brightness. Fine grain and homogeneous crystal structure produce a brighter finish. Of particular note is the finding that barreltumbled or shot-blasted parts brighten more brilliantly and more uniformly than do castings without this preliminary treatment. This is due to the work hardening or breaking up of crystals on the surface of the metal. Thus one use of the invention will be the use of the composition for brightening small zinc castings which have been barrel-tumbled. I

The greatest use will be for small, rounded, zinc castings such as automotive parts (carburetor parts, handles) small tool parts, toys, etc., wherein an economical bright finish is desired.

The novel bright dip bath used in the present invention is composed of hydrofluoric acid, chromic acid (added as chromic anhydride-CrOQ and nitric acid. The following range of proportions have been found to give excellent results:

Usually the ratio of the weight-percentage of hydrofluoric acid (HF) to nitric acid (HNO is approximately of the order of 1:2. A very wide range of composition is possible and the above range is not altogether limited as given, but is only illustrative of the wide range of acid formulations which are possible.

The time of immersion in the bath varies with the temperature. For instance, at about F., it might take three minutes or more to produce the desired brightness, whereas at 170 F., only a few seconds are necessary. The temperature range which has given the best results has been about -150 F.

Air agitation or mechanical agitation of the bright dip solution is very beneficial. Container materials to hold the solution may be acid-resistant plastics, Koroseal, Saran, or polyethylene compounds, and carbon bricks. Karbate (carbon) heating coils are used for heating the solution. For definitions of these trademarked prodwanna ucts see Condensed Chemical Dictionary, 3rd edition (1942), pages 383, 564, and 380, respectively.

Broadly, the invention comprises the following steps: cleaning and/or tumbling the zinc castings; placing the cleaned parts in the novel bath, which is maintained at the temperature where the speed and degree of brightness is desired; leaving the objects in the bath until they have been deburred or brightened to the desired degree; and lastly, removing the objects from the bath and rinsing and drying them.

In carrying out this process, the manner in which the articles are cleaned and/or tumbled, the operating temperature, and length of time in the bath may be varied to suit the particular alloys being treated without departing from the invention. An example of operations used to carry out the process consists of the following steps:

(1) Clean objects or barrel-tumble them. The cleaning operations may consist of mild alkaline cleaning or solvent degreasing.

(2) The zinc objects are rinsed thoroughly, if they have been cleaned by alkali.

. (3) Next, the zinc objects are immersed in the novel treating bath for a sulficicnt period to bring about the deburring or brightness of surface. The optimum temperature range is 140-150 F. A typical formula for such use is:

Chromic acid flakes =8.45 fld, oz./gal. Water to make 1 liter 1 gallon (4) The objects are then thoroughly rinsed in cold and then in hot water.

(5) Finally, the objects may be dried with warm or hot air. The objects may be processed in bulk in plasticcoated baskets or treated as individual parts. Movement of the parts in a basket, while being processed, is very beneficial.

While the above procedure produces excellent results, it should be understood that the invention is not confined solely to the above exact steps.

The dip solution may be used on severely oxidized zinc alloys, such as zinc base die castings, to impart a bright, clean finish suitable for plating or painting or other chemical treatments.

The novel bright dip can be used as a final bright surface in itself. It shall find use in producing an economical finish for small tool parts, toys, automotive parts, etc.

Zinc cast objects usually contain oxide inclusions, etc., which interfere with a subsequent electrodeposit.

' Bright dipping such parts produces a better adherent electroplated coating and lessens diffusion oi. the applied coating when heated.

The novel bright dip brightens areas of objects which could not be brightened by mechanical means such as polishing or bufling.

The novel bright dip imparts some protection to the treated surface by leaving a slight chromate film on the surface. Treated zinc base die castings have withstood 24 hours of continuous salt spray testing without showing white corrosion products from the salt spray.

The passive film produced on the objects imparts a good base for subsequent paints or clear lacquers. For

. instance, lacquered, untreated zinc base die castings broke down in salt spray tests around 24 hours, whereas castings treated with the novel bright dip and then subsequently lacquered, withstood 89 hours of a salt spray.

The novel d-ip may be used for a deburring operation or fine scratch-line removal.

The following table gives a number of compositions falling within the scope of the present invention:

Table I Chromic Nitric Acid Hydro- Acld (ClOa, HNOa(1.42 fiuorieAcld Tempera- Composltion flakes sp. gr), (48% by ture, F.

Grams/liter mLIhter Wei ht), ml. ter

1 This dip worked fairly well on Zamak" #5 castings. I Good results for "Zamak #2 all Agitation was used throughout.

In all of the above formulations, the ingredients are made up to one liter by the addition of water.

I claim:

1. Method of imparting a bright surface finish to zinc alloys containing 80% and upwards of zinc which comprises immersing them for a short time in an aqueous solution having the following composition, by weight:

Water sutficient to make 100%.

2. The method as defined in claim 1 in which the ratio of HP to HNO is approximately as 1:2.

3. Method of imparting a bright surface finish to zinc alloys containing 80% and upwards of zinc which comprises immersing them for a short time in an aqeous solution containing about 100 grams per liter of chromic oxide, 100 milliliters per liter of concentrated nitric acid having a specific gravity of about 1.42, and about 65 milliliters per liter of 48% hydrofluoric acid.

4. The method as defined in claim 3 in which the solution has about the following composition:

5. A composition of matter for imparting a bright lustrous finish to articles made of a zinc alloy, comprising by weight Percent Chromic anhydride (Cr0 6.5 to20.0 Nitric acid (HNO 2.7 to 15.0 Hydrofluoric acid (HF) 1.0to 8.0

Water sufiicient to make 100%.

6. A composition of matter for imparting a bright lustrous finish to articles made of a zinc alloy, comprising CrO (chromic acid flakes) grams 100 HNO (concentrated nitric acid, specific gravity 1.42) milliliters 100 HF (hydrofluoric acid containing 48% of actual HF) 'milliliters 65 Water to make 1 liter.

References Cited in the file of this patent UNITED STATES PATENTS 2,186,579 Dupernell et a1. Jan. 9, 1940 2,593,448 Hesch Apr. 22, 1952 

1. METHOD OF IMPARTING A BRIGHT SURFACE FINISH TO ZINC ALLOYS CONTAINING 80% AND UPWARDS OF ZINC WHICH COMPRISES IMMERSING THEM FOR A SHORT TIME IN AN AQUEOUS SOLUTION HAVING THE FOLLOWING COMPOSITION, BY WEIGHT: 